1. Field of the Invention
The present invention relates to a method for dumping and disposing of carbon dioxide gas and an apparatus therefor, and more particularly to a method for dumping and disposing of carbon dioxide gas exhausted from industrial furnaces and an apparatus therefor.
2. Description of the Related Art
Since carbon dioxide gas exhausted from various sorts of industrial furnaces such as furnaces in thermal power plants has increased the temperature of the atmosphere on the earth in recent years, it becomes a great problem for mankind to prevent the temperature of the atmosphere from rising. As a method for preventing the temperature of the atmosphere on the earth from rising, a method for dumping carbon dioxide gas has recently been disclosed.
A method for dumping carbon dioxide in some places other than in the atmosphere to prevent temperatures on the earth from rising involves dumping carbon dioxide separated from gases exhausted from thermal power plants and the like into the ocean, disused oil wells, dead pits of rock salt or the like. This method is shown in "A system study for the removal, recovery and disposal of carbon dioxide from fossil fuel power plants in the U.S." (M. Steinberg et al., Rep. Brookheaven National Laboratory, BNL 35666).
Dumped carbon dioxide cannot be permanently fixed to a place, where the carbon dioxide is dumped, by using the method for dumping carbon dioxide into the ocean, dead pits or the like.
Japanese Patent Application Laid Open No. 80316/90 discloses a method for dumping carbon dioxide gas into the ocean. The method involves dumping carbon dioxide gas to a depth of more than 700 m under the sea surface where it is dissolved in seawater upto a saturated concentration. In this system, advantage is taken of the fact that an amount of dissolved carbon dioxide gas in the ocean has not attained a saturated value, and of the fact that the amount of dissolved carbon dioxide gas in the seawater at the depths of the ocean is small.
When carbon dioxide gas is just dumped into the ocean to be dissolved in seawater, a problem arises concerning the dissolving rate of carbon dioxide gas in seawater. The dissolving rate of carbon dioxide gas decreases with an increase of the concentration of carbon dioxide gas which is dissolved in the seawater. The concentration of carbon dioxide gas is a ratio of an amount of dissolved carbon dioxide gas to the saturated amount of dissolved carbon dioxide gas. The concentration of carbon dioxide gas adjacent to where carbon dioxide gas is dumped, that is, adjacent to where an apparatus for injecting carbon dioxide gas is located, increases as a result of the carbon dioxide gas being injected into seawater. The dissolving rate of carbon dioxide gas therefore decreases with the lapse of time during which the apparatus for injecting carbon dioxide gas operates. Accordingly, the apparatus for injecting carbon dioxide gas is preferably installed at a place where the tide runs strong so that carbon dioxide can be dumped into "fresh" seawater having a low concentration of dissolved carbon dioxide gas. However, since the tide at the depths of the sea, which is considered favorable for dumping carbon dioxide gas, runs weak, the aforementioned method has a problem in that the dissolving rate of carbon dioxide gas becomes lower with the lapse of time during which the apparatus for injecting carbon dioxide gas operates.
It is feared that dumping of carbon dioxide into the ocean has a bad influence on oceanic life and gives rise to destruction of oceanic environment. Japanese Patent Application Laid Open No. 133308/90 discloses methods for decreasing the bad influence on the environment of a sea area, into which the carbon dioxide gas is dumped. One of the method described in this publication is a method wherein a pressure vessel is filled up with liquefied carbon dioxide gas to be dumped and the pressure vessel is dropped into the sea. Another method disclosed in the publication is a method wherein an empty vessel is buried at the bottom of the sea and then filled with liquefied carbon dioxide gas.
The method wherein a pressure vessel filled up with carbon dioxide is dropped into the ocean and wherein carbon dioxide gas is pumped into a vessel buried at the bottom of the sea are not appropriate for measures to counter pollution wherein a great amount of exhaust gas must be disposed of.
A method for dumping carbon dioxide gas, which is disclosed in Japanese scientific magazine "Kagaku Asahi" (Vol. 12, 132-133, 1990), involves dumping liquefied carbon dioxide gas to a depth of more than 3000 m under the sea surface where a water pressure is more than about 30 MPa. At this pressure, the density of the liquefied carbon dioxide gas becomes larger than that of the seawater and it is anticipated therefore that the liquefied carbon dioxide gas will accumulate in depressions of the sea bottom, and that any seawater, into which the carbon dioxide gas has dissolved, will spread or creep over the sea bottom to be neutralized at places where calcium carbonate has accumulated. The reaction is expressed as follows: EQU CaCO.sub.3 +CO.sub.2 +H.sub.2 O.fwdarw.Ca.sup.2+ +2HCO.sub.3 -
The method disclosed in the magazine relies on gas hydrate formation at a contact area where an upper portion of layers of liquefied carbon dioxide gas accumulated in depression of the sea contacts the seawater; a gas hydrate is a solid like ice. The gas hydrate is represented by the molecular formula: CO.sub.2 nH.sub.2 O. The gas hydrate can act as a cover to contain and prevent liquefied carbon dioxide gas from dispersing.
The gas hydrate is a solid wherein water molecules from polyhedra called "host structure" and a gas molecule is included as "guest molecule" in each of the polyhedra. This sort of clathrate hydrate and simply a clathrate as well as the gas hydrate. Hereinafter, this compound is referred to as the gas hydrate. It depends on temperature, pressure and concentration of impurities whether the gas hydrate exists stably or not. Minimum pressures necessary for the stable existence of the gas hydrate have been already made clear as a function of temperature by numerous studies. Results on many sorts of gas hydrates as well as the gas hydrates of carbon dioxide are shown, for example, in "Clathrate Hydrate in Water --A Comprehensive Treatise" (D. W. Davidson, Vol. 2 ed F. Franks, Plenum Press, 1972). Stabilities of gas hydrate will be shown later in this document.
Even if carbon dioxide is dumped into the seawater at depths below 3000 m, the dumped carbon dioxide is released again into the atmosphere because of the following seawater current on a global scale. The current flows toward the equator from a polar region at a bottom layer of the ocean and ascends at an equator area. Therefore the seawater containing dumped carbon dioxide should appear at the sea surface around the equator. Because the solubility of carbon dioxide into the seawater decreases with an increase of temperature, it is supposed that the super-saturation is attained at the sea surface, which results in the release of carbon dioxide contained in the atmosphere because of the dumping of carbon dioxide into the atmosphere.
Furthermore, the dumping of carbon dioxide into the deep sea must be carried out by passing the carbon dioxide through a steel pipe at high pressures of more than 30 MPa, which requires an enormous cost.
It should be noted that it is for making the density of liquefied carbon dioxide gas greater than that of seawater that the pressure at a great depth of the sea is required, but such great pressure is required not to liquefy carbon dioxide gas or to form the gas hydrate. For example, the pressure necessary for liquefying carbon dioxide gas and the pressure necessary for forming the gas hydrate are about 4 MPa and about 2.3 MPa respectively, when the temperature of the seawater is 5.degree. C. This is an order of magnitude less than is required by the method described in "Kagaku Asahi" (Vol. 12, 132-133, 1990) discussed above.
To see the relation between the depth of the sea and the existing conditions of carbon dioxide, representative vertical distribution of temperatures of seawater and a phase diagram of carbon dioxide overlapping each other are shown in FIG. 5. In FIG. 5, curve A, B and C denote the vertical distribution of temperatures of seawater, the saturated vapor pressure of carbon dioxide and the dissociation pressure of the gas hydrate, respectively. From the saturated vapor pressure of curve B and the dissociation pressure of the gas hydrate of curve C, it can be seen that the depth at which the condition necessary for liquefying carbon dioxide or for forming the gas hydrate occur is about 400 m.
It is well known that the gas hydrate is formed not only by carbon dioxide, but also by natural gas. In addition to dumping carbon dioxide, the present invention provides a means whereby the dumping of carbon dioxide can be used for recovery of natural gas from a layer of natural gas hydrate in nature. Scientific and technical papers related to the natural gas hydrate is therefore reviewed in the next place.
FIG. 13 is a graphical representation showing the relation between the temperature and the dissociation pressure of the gas hydrate relative to methane being major component of natural gas; the dissociation pressure of carbon dioxide is also shown in the figure to compare both hydrates. In the drawing, curve (a) denotes an equilibrium relation among carbon dioxide gas, hydrate of carbon dioxide gas and water or ice and curve (b) shows an equilibrium relation among liquefied carbon dioxide gas, carbon dioxide gas and hydrate of carbon dioxide gas, that is the saturated vapor pressure as a function of the temperature. Curve (c) denotes an equilibrium relation among methane gas, methane gas hydrate and water or ice. Dissociation pressures of methane hydrate and carbon dioxide at 0.degree. C. are 2.6 MPa and 1.2 MPa, respectively. When the pressure is elevated, both hydrates are stably present even at temperatures greater than 0.degree. C.; the dissociation pressures of methane hydrate and carbon dioxide are 6.9 MPa and 4.5 MPa at 10.degree. C., respectively. In the case of carbon dioxide, liquefied carbon dioxide gas and the gas hydrate can coexist. On the other hand, since the critical temperature of methane is low (-82.2.degree. C.), methane gas hydrate can coexist with only methane gas at temperatures from -10.degree. to +12.degree. C. shown in FIG. 13.
There is a substantial amount of the natural gas hydrate in the natural world. According to Kamath et al., it is confirmed that the natural gas hydrate exists not only in the oil field regions along the coast of the Gulf of Mexico, but also in the permafrost regions and oil field regions along the coasts of Alaska, Canada and Siberia (see e.g. "Cold Regions Science and Technology", Vol. 14, 107-119, 1987).
It is not a long ago that the existence of the natural gas hydrate was found in the natural world. According to Ryo Matsumoto, the natural gas hydrate was found for the first time in the permafrost regions of Siberia in 1970. This is shown in his paper "Nature and occurrence of gas hydrate and their implications to geologic phenomena", (Journal of geological society of Japan, Vol. 93, 597-615, 1987). It is presumed that an enormous volume of natural gas hydrate is present under ground 200 to 1000 meters deep from the earth's surface. In 1980, the natural gas hydrate was recovered in large quantities in Blake Outer Ridge off Florida according to "Deep Sea Drilling Project". It was understood according to this project that a boundary between a layer of the natural gas hydrate and soil could be clearly distinguished by means of seismic exploration. Thereafter, this boundary was confirmed throughout the world. It is conceivable from this that an enormous volume of the natural gas hydrate is present under ground at the sea bottom.
The natural gas hydrate has a great possibility as an energy resource. According to an estimate of reserves of the natural gas hydrate, 1.1.times.10.sup.4 giga tons of the natural gas hydrate exist underground at the sea bottom and 4.times.10.sup.2 giga tons under ground of permafrost regions as a mass of carbon contained in the natural gas hydrate. This is shown in "Role of methane clathrates in past and future climate, Climate Change", (G. J. Macdonald, Ann. Rev. Energy, Vol. 16, 247-281, 1990). An estimate that a total energy of reserves of the natural gas hydrate is five times larger than that of coal reserves is pointed out. This is shown in "The future of methane as an energy resource" (G. J. Macdonald, Vol. 15, 53-83, 1990).
A method for gathering natural gas from layers of the natural gas hydrate is disclosed in Japanese Patent Application Laid Open No. 172094/82. The method of this publication is illustrated in FIG. 12. As shown in FIG. 12, each of the lower ends 224, 224 (a) of pipe bodies 222 and 223 respectively, is penetrated into layers 225 of the natural gas hydrate. The upper ends 222 (a), 223 (a) are positioned above the surface 221 of the sea. Seawater is poured through an inlet port 226 into the pipe body 222 to outlet 224 (a) into the natural gas reservoir. The natural gas hydrate is heated by the seawater whereby natural gas 227 is separated from the hydrates. The natural gas 227 and seawater enter inlet 224 and rise through pipe body 223. The seawater is discharged from an outlet port 228 into the sea. The natural gas 227 rises through the pipe body 223 and is recovered by gas holder on the sea.
This method disclosed in the Japanese Patent Application Laid Open No. 172094/82 wherein gas is recovered by injecting seawater into layers of the natural gas hydrate has a difficulty in that this method cannot be executed in permafrost regions where there is no easy access to seawater or water.